1. Field of the Invention
The invention relates generally to CMOS image sensors. More particularly, the invention relates to backside illuminated CMOS image sensors with pinned photodiodes.
2. Description of Related Art
Charge-coupled device (CCD) and complementary metal-oxide-semiconductor (CMOS) image sensors are two different technologies used for capturing images digitally. Both types of imagers convert light into electric charge. A CCD sensor has charges transferred from every pixel to a limited number of output nodes for conversion to voltage, while CMOS sensors have charge-to-voltage conversion for each pixel.
Conventional front side illuminated CMOS image sensors suffer from a number of drawbacks. For example, reduced photo-response, obscurations from metal lines crisscrossing pixel area, reduced fill factor, low short and long wavelength quantum efficiency (QE) for blue photons and near-infrared (NIR) wavelengths, respectively, and interference fringing from thin passivation and interlayer dielectrics.
Backside illuminated CCDs have active pixel circuitry, such as electrodes and gates, arranged on the front surface of each substrate wafer. The backside is illuminated to circumvent preferential absorption of blue photons that typically result from front side illumination and to increase the absorption of NIR photons.
Conventional backside illuminated CCDs also have a number of drawbacks. For example, backside illuminated CCDs lack single chip system capabilities. Conventional backside illuminated CCDs are also not suited for high frame rate low noise imagers.
Using CMOS backside illumination, the image sensor operates with no metal line related effects and with up to 100% fill-factor. Backside illuminated CMOS image sensors provide high quality QE, excellent angular response and availability of additional space in the pixel for integration of in-pixel signal processing circuits. Despite these enumerated advantages, the backside illuminated CMOS image sensors suffer from a number of drawbacks. Backside illuminated CMOS image sensors with conventional photodiodes have high noise, high dark current and high sense node capacitance.
With an ever increasing demand for improved image sensors, there remains a need in the art for backside illuminated CMOS image sensors with reduced noise, dark current and sense node capacitance.